The diamine putrescine and the polyamines spermidine and spermine are ubiquitous in animal tissues and are important compounds in growth and differentiation. Studies concerning the origin, fate, regulation and metabolic role of circulating polyamines are being continued. In addition, metabolism of (14C)putrescine was studied in sliced rat liver. A new metabolite, 2-pyrrolidone, was found to be the major product of putrescine metabolism in incubation medium.